Present day food stores such as supermarkets and convenience stores typically use relatively high capacity refrigeration systems to keep their refrigerated and frozen food products cold. The two most common types of refrigeration systems may be generally designated as direct expansion systems and secondary coolant systems. In direct expansion systems, a two-phase, vapor-compression refrigeration loop is used which normally includes an evaporator positioned inside the refrigerated space that absorbs heat from the space, thereby cooling the space to the desired temperature. In secondary coolant systems, a primary refrigeration loop and a secondary refrigeration loop are used in conjunction to cool the refrigerated space. The primary loop of the system is typically a vapor-compression system similar to that used in direct expansion systems and usually comprises a compressor, condenser, receiver, and an expansion device. The secondary loop is typically a single-phase system and comprises a pump and a heat exchanger that is disposed within the refrigerated space to absorb heat therefrom. The two loops of secondary coolant systems thermally communicate with each other through a chiller which provides for heat transfer between the primary and secondary loops.
Currently, there is a trend toward use of secondary coolant systems rather than direct expansion systems in that the amounts of primary refrigerant used in the refrigerated space can be minimized when a secondary coolant system is used, increasing safety to personnel and customers that interact with the refrigerated space. In addition, secondary coolant systems provide the advantage of improving temperature stability and humidity within the refrigerated space.
As is well known in the art, moisture contained within the refrigerated space condenses on the heat exchanger used in the refrigerated space and freezes thereon to form frost. This frost greatly decreases the cooling efficiency of the refrigeration system and, if left to accumulate, can even block the flow of air through the evaporator or heat exchanger to diminish the heat exchange capacity of the refrigeration system. Several methods of removing this frost, known as defrosting, have been developed in the refrigeration arts. The simplest method is so called "off-cycle" defrost in which the refrigeration cycle is simply discontinued and the heat of the surrounding air meets the frost. In another method, the evaporator or heat exchanger is electrically heated to melt the frost. In direct expansion systems, typically the hot gas of the refrigerant discharged by the compressor is used to melt the frost. In yet another method, the secondary coolant system is defrosted by passing warm coolant through the refrigerated space heat exchanger for a predetermined period of time and/or temperature, so that the frost formed thereon melts and drains away. Of these several methods, liquid defrost is generally preferred in the art for several reasons. First, warm liquid defrost is safer than electrical and hot gas defrost in that it is less stressful on the refrigeration system. In addition, warm liquid defrost is more efficient than electrical and hot gas defrost and therefore does not result in a large degree of warming of the refrigerated space. This avoids food spoilage and also increases system efficiency in that a large degree of cooling is not necessary to bring the refrigerated space back to its standard operating temperature.
The most common methods of heating the liquid supplied to the coils located in the refrigeration space typically utilize the hot gas of the refrigeration system that is discharged by compressor. In particular, the hot gas from the compressor is diverted to a gas-to-liquid heat exchanger, often referred to as a heat reclamation tank, in fluid communication with the secondary coolant in which the coolant is heated so it then can be delivered to the refrigerated space heat exchanger.
Although typically providing enough heat energy to adequately defrost the coils of the refrigerated space evaporator or heat exchanger, usage of gas-to-liquid heat exchange presents several disadvantages. Specifically, gas has a relatively low coefficient of heat transfer in comparison to liquid. Due to this relatively low coefficient of heat transfer, the defrost liquid often must be prepared in advance of the defrost cycle to ensure adequate heating of the refrigeration space coils. Accordingly, defrost in many systems cannot be had "on demand." Moreover, the relatively low coefficient of heat transfer of the gas mandates relatively large heat transfer surface areas between the gas side and the liquid side of the heat reclamation tank or other heat exchanger. To provide this large heat transfer surface area, the heat reclamation tank or other heat exchanger typically must be large in size and, consequently, is quite expensive. Additionally, usage of heat reclamation tanks often requires the usage of other expensive equipment such as valves and control systems which are used to control operation of the reclamation tank.
From the above, it can be appreciated that it would be desirable to have a refrigeration system which utilizes warm liquid defrosting of the refrigerated space coils which is not dependent upon the hot discharge gas from the compressor and gas-to-liquid heat exchange.
Where the refrigeration system comprises a medium temperature side for cooling refrigerated goods and a low temperature side for cooling frozen goods, typically the discharge gases of the medium temperature and low temperature sides are used separately to warm the coolants of the medium temperature and low temperature sides, respectively, for defrost.
Although typically providing enough heat energy to adequately defrost the coils of the refrigerated space evaporator or heat exchanger, separate coolant warming on the medium and low temperature sides can be inefficient. This inefficiency is apparent when the individual heating capacities of the medium and low temperature sides are analyzed.
Because of the different respective temperatures needed in the refrigerated spaces of the medium and low temperature sides, the temperature of the warm liquid needed for defrost typically is different for these two sides. For example, a refrigerated space may require coolant at a temperature of approximately 20.degree. F. flowing through the refrigerated space heat exchanger and exiting at 25.degree. F. to maintain the desired temperature therein. While the low temperature refrigerated spaces may require a coolant at a temperature of approximately -20.degree. F. and exiting at approximately -15.degree. F. to maintain the desired temperature therein. These respective temperatures mean that typically approximately 50.degree. F. to 55.degree. F. coolant is needed for defrost on the medium temperature side while typically approximately 70.degree. F. to 75.degree. F. coolant is needed for defrost on the low temperature side. Accordingly, the temperature change required to heat defrost liquid for the medium temperature side is approximately 30.degree. F. (the difference between 25.degree. F. and 55.degree. F.) while the temperature change required to heat defrost liquid for the low temperature side is approximately 90.degree. F. (the difference between -15.degree. F. and 75.degree. F.).
From the above, it can be appreciated that the temperature change of the coolant needed for defrost on the low temperature side is three times that needed on the medium temperature side. Typically, however, most refrigeration applications require three times as much medium temperature cooling as low temperature cooling. This means that the medium temperature side of the refrigeration system must have three times the mass flow of the low temperature side. Accordingly, conventional systems typically have a low temperature side with only one third the heating capacity of the medium temperature side, but which requires three times the temperature change of coolant for defrost. It is this uneven balance of heating capacity and required temperature change that creates the aforementioned inefficiency of conventional refrigeration systems.
It therefore can be appreciated that it would be desirable to have a warm liquid refrigeration system which utilizes the relatively large heating capacity of the medium temperature side of the refrigeration system to heat the defrost liquid for the low temperature side of the refrigeration system.